1. Field of the Invention
The main technical field of the present invention is the medical field, and the present invention relates to an x-ray three-dimensional imaging method and apparatus.
2. Prior Art
Diagnosis by means of X-ray photographs is indispensable in modem medicine. In particular, X-ray tomography (hereafter referred to as xe2x80x9cX-ray CTxe2x80x9d) provides information that is extremely useful for the diagnosis of pathological changes, etc. that occur inside the cranium and in the abdomen. In addition, such tomography is used in various other applications, e.g., as one method of so-called non-destructive examination used to investigate the internal structure of cultural materials without destroying such materials.
In a conventional X-ray CT apparatus, imaging is performed by causing an X-ray tube to rotate along the ring of an X-ray detector arranged in an annular (one-dimensional) configuration. Accordingly, the X-ray detector can only obtain information for a specified section (sectional plane) of the human body constituting the object of imaging. As a result, the image that is obtained is a simple planar image, and is not suitable for obtaining a three-dimensional understanding of the structure of the object of imaging.
For example, in cases where it is desired to obtain an overall image of a certain organ, respective sectional images are obtained by setting sectional planes at several locations with respect to the organ; then, these sectional images are read by a reader (physician), and the structure of the organ is mentally envisioned. Accordingly, considerably skill and experience are required in order to comprehend an overall image of an organ by reading such images. On the other hand, in cases where a spiral X-ray CT developed in recent years is used, the X-ray tube moves in a spiral configuration so that a plurality of different sectional planes can be continuously imaged under the same conditions. Since these sectional images are stored as image information inside a computer, image processing such as the preparation of sectional images from any desired direction, the extraction and display of specified tissues organs only, or the preparation of views traced from any desired direction, etc. can be performed following imaging.
However, in cases where such a spiral X-ray CT device is used, considerable time is required for imaging, and drawbacks arise. Namely, the exposure of the patient to X-rays is increased, and there is an increased burden on the patient due to the fixed posture required. Furthermore, for organs whose movement cannot be voluntarily stopped by the patient, such as the heart, etc., clear images cannot be obtained.
Recently, therefore, methods for imaging a plurality of sectional images more efficiently than in the case of spiral CT have been proposed. In Japanese Patent Application 5-517284, a means for the efficient two-dimensional detection of X-rays is discussed, and a means for simultaneously obtaining a plurality of sectional images merely by causing the X-ray tube to perform a single circuit around the object of imaging is indicated. Furthermore, in Japanese Patent Application Laid-Open (Kokai) No. H 6-233757, clearer images are obtained by forming the irradiating X-rays into a parallel beam.
However, in the case of the former device, as sectional images that are vertically separated from the circling track of the X-ray tube are imaged, the angle formed by the sectional images and the X-rays becomes mathematically difficult to supplement, so that the sectional images become blurred. Though this problem does not occur in the latter device, there are many technical problems with regard to the device that generates a parallel X-ray beam, so that this system has not yet reached the stage of practical use. In any case, these methods are merely methods for the efficient imaging of a plurality of sectional images of the object of imaging. As evidence of this, a point that is common to both inventions is that the X-ray tube must move in a circuit around the object of imaging. When such a moving system is adopted for the X-ray tube, the size of the overall apparatus is increased, and the manufacturing method cost is also increased. Furthermore, installation, operation and maintenance of the apparatus also become difficult.
More specifically, since the X-ray tube and X-ray detector rotate about the human body at a high speed, the centrifugal force is proportional to the rotational radius, and increases in proportion to the square of the angular velocity. Accordingly, if these devices perform one circuit about the human body in 0.1 seconds, a centrifugal force that is close to 200 times the force of gravity is generated. In such a case, many technical problems occur that involves, among others, a strong mechanical strength is required in the parts that fasten the apparatus in place, a powerful driving device is required, and the apparatus must be absolutely free from vibration.
The present invention solves the above-described problems. More specifically, in the surveying of the X-ray transmission coefficients of an object of imaging, the present invention makes it possible to obtain a means that measures only the X-ray transmission coefficient at an arbitrary point in the object of imaging (which has a three-dimensional extension) independently of other points, with absolutely no need for setting sectional planes. Furthermore, the X-ray transmission coefficients at all points within the object of imaging, i.e., an X-ray three-dimensional image, can be imaged by a series of scans in the same manner as the single-image imaging of a conventional X-ray CT.
In addition, since it is not absolutely necessary for the X-ray tube to rotate about the human body during imaging, and since the X-ray detector can be fixed, the weight of the movable parts can be greatly reduced. Furthermore, as will be described later, the series of scans can be completed in an extremely short time by magnetically controlling the position at which the X-rays are generated. As a result, three-dimensional images that show no blurring can be obtained even in the case of organs that move involuntarily, such as the heart, etc. Accordingly, by imaging numerous three-dimensional images over time, it is possible to image the movement of the organ itself as a movie image.
In accordance with the above-described concept, the present invention provides an X-ray three-dimensional imaging method that comprises the following operations 1) through 4):
1) an operation in which a step of irradiating an object of imaging with X-rays from an X-ray tube, detecting a planar density of radioactivity of X-rays passing through the object of imaging by an X-ray detector in which very small pixels are arranged in a planar configuration, and obtaining such radioactivity density as an X-ray original image is performed with at least positions of X-ray irradiation changed, thus obtaining X-ray original images at respective X-ray irradiation positions;
2) an operation in which irradiation from an X-ray irradiation position in a specified position is performed with respect to a certain observation point in the object of imaging, and an X-ray transmissivity obtained by dividing planar density of radioactivity of the X-rays passing through the observation point by planar density of radioactivity in a case where it is assumed that no object of imaging is present is obtained from pixels determined from positions of the irradiation position and observation point within the X-ray original image corresponding to the irradiation position;
3) an operation in which the transmissivity of the X-rays passing through the observation point is determined by performing the operation 2) from all other X-ray irradiation positions, and an X-ray transmission coefficient at the observation point is obtained by subjecting transmissivity values to an averaging treatment; and
4) an operation in which the operations 2) and 3) are performed for all observation points in the object of imaging so that the X-ray transmission coefficients at respective observation points are determined, and distribution of the X-ray transmission coefficients in the object of imaging is obtained as an X-ray three-dimensional image.
Furthermore, the present invention provides an X-ray three-dimensional imaging apparatus that performs the method described above, and the X-ray three-dimensional imaging apparatus comprises at least: an X-ray tube which is capable of irradiating the object of imaging with X-rays and whose X-ray irradiation positions and direction can be quickly changed; an X-ray detector in which very small pixels that can detect the X-rays passing through the object of imaging and output a physical quantity that corresponds to a planar density of radioactivity of the X-rays are arranged in a planar configuration; a central processing unit which controls an operation of an X-ray tube and an X-ray detector, and which determines X-ray three-dimensional image by processing a detection data of the X-ray detector; and an output device which outputs a three-dimensional image determined by the central processing unit.
More specifically, in the present invention, the amount of information obtained per unit time is greatly increased by arranging the X-ray detector in a planar configuration instead of a linear configuration as in conventional systems. In a conventional X-ray CT device, the X-ray detector is arranged in a linear (one-dimensional) configuration. Accordingly, information (X-ray dosages) in a two-dimensional space (in mathematical terms) is collected in combination with information concerning the rotational angle of the X-ray tube (one-dimensional information). Consequently, the images that are obtained are also limited to sectional images as two-dimensional images. The present invention is characterized by the fact that the X-ray detector is arranged in a two-dimensional (planar) configuration, so that three-dimensional images, i.e., X-ray three-dimensional images can be constructed by expanding the measurement space to three dimensions.
Furthermore, as examples of application in conventional X-ray CT devices, there have been devices that are designed so that a plurality of sectional planes can be simultaneously imaged using a two-dimensional detector similar to that of the present invention as the X-ray detector. However, as described above, such devices are outside the scope of the present invention. The present invention differs from these devices in that the X-ray transmission coefficients at all points within the object of imaging can be independently and simultaneously confirmed.